a(n) = (n/2)*binomial(n-1, floor((n-1)/2)) - 2^(n-2).

Terms

a(0) =0a(1) =0a(2) =1a(3) =2a(4) =7a(5) =14a(6) =38a(7) =76a(8) =187a(9) =374a(10) =874a(11) =1748a(12) =3958a(13) =7916a(14) =17548a(15) =35096a(16) =76627a(17) =153254a(18) =330818a(19) =661636a(20) =1415650a(21) =2831300a(22) =6015316a(23) =12030632a(24) =25413342a(25) =50826684a(26) =106853668a(27) =213707336a(28) =447472972a(29) =894945944